Science Corner: VPD

by Stephen Brookes

How does the vapor pressure deficit impact a plant’s growth and the quality of the harvests?

In its most basic form, VPD is the difference between the amount of water (vapor) in a room and how much water the room can hold when it’s saturated. Knowing the VPD is essential because, at low pressure, moisture can form on the plant’s leaves and fruits, increasing its susceptibility to mold and rot. At higher pressure, the plant needs to increase its uptake of water to keep up with its water losses to the room (transpiration). When this happens, the stomata (small pores on the underside of a leaf that take in water and CO2) start to close, and the plant will stop growing.

Why Bother With VPD?

In 2017, Zhang, D published an article on VPD in greenhouses showing how proper control can reduce stress and help regulate irrigations to save water over a tomato growing season (1). In indoor grow rooms, monitoring and regulating VPD will help solve many problems during the vegetative and flowering cycles. Furthermore, it will also help you achieve show-grade fruits, vegetables, and herbs.

Firm fruits, glossy and colorful herbs, and flavorsome vegetables are a product of the environment they are grown in and have very little to do which the nutrients used. In other words, there is truth to the statement that ‘environment is everything’. When you control VPD, you are allowing the plant to uptake water, nutrients, and CO2 in the most efficient way possible, which speeds up metabolism for faster growth and enables the plant to reach its genetic potential for yield and quality.

How To Measure VPD

You’ll need a monitoring system, such as the Minder from PL Grow systems, which can tell you the temperatures, humidity, and VPD from anywhere in the world. These units are small, compact, and accurate. If newer technology intimidates you, have a thermometer/hygrometer and an infra-red thermometer handy to measure the plant leaf temperature. Plug the information into a chart to work out VPD, such as this one from Omni: bit.ly/31LZnB2.

VPD charts generally all look the same, but it’s essential to pick one that doesn’t overcomplicate things. Google search ‘VPD charts’ and click on the images of one that is color-coded and looks simple to follow.

The chart shows that if you manage humidity as the temperature increases or decreases, you can keep the VPD in the green zone (safe) and allow your plants to grow happily. The same applies to changes in humidity, where you can increase or decrease the temperatures to keep everything moving along at a good pace.

The numbers in each square may be confusing at first, but they represent the pressure plants are experiencing in kPa (Kilopascal). The numbers are easy when you know what they should be in your room.

Propagation and early veg: 0.8 kPa (+/- 0.2) Late veg and early flower: 1.2 kPa (+/- 0.2) Mid to later flower: 1.4 kPa (+/- 0.2) These are the basics; once you are happy with the performance in your grow room, you should naturally begin to learn more and explore VPD in detail, such as the leaf temperatures. In a plant that is metabolizing well and in good health, the leaf temperature should be 2-3°C lower than the room temperature. Monitor the temperatures, humidity, and VPD with a data logger consistently over 24 hours and make sure the plant is in the optimum environment during the lights on and the lights off stages. VPD is critical to achieving nutrient-dense fruits and high yields.

How To Control VPD

To control VPD, you need to know what the readings are, so whichever method you use (technology or manual measuring), be accurate and consistent with the measurements.

Controlling the humidity is where most growers will want to go first to manipulate the VPD. Consider investing in good humidifiers and dehumidifiers with controllers. With these two items, you can maintain a steady relative room humidity, and therefore, have reasonable control over VPD.

The control factor can become more challenging during the summer (with plenty of moisture in the air) as well as in the winter (dry air). Altering the room temperatures will help with that issue. Most growers will do this by increasing or decreasing airflow, dimming or boosting the lights, and changing the time of day the lights are on and off. There are hundreds of articles and podcasts that go into more detail on this if you need more information.

Conclusion

Too many growers are still spending money on the ‘secret nutrient’ that increases their yields or improves quality. Go back to the basics with nutrients, and concentrate on creating the perfect environment with particular attention to the VPD. Instead of buying fancy nutrients, purchase the right hardware to take measurements. Whether it’s better quality, more vivid colors, or an increase in yields, I promise this will have a far more significant impact on your grow. 3

Bio

Stephen Brookes is a hydroponics aficionado and loves to apply the scientific method to his articles. He has been the manager of NPK Technology for 10 years, and produces and hosts the world’s number one hydroponics podcast, NPK Live. Stephen is also owner of NPK Media, a 360 media content production agency. He likes to read and enjoys mountaineering in his spare time. Motto: The more you learn, the less you know.

Reference: (1) Zhang, D., Du, Q., Zhang, Z., Jiao, X., Song, X. and Li, J., 2017. Vapor pressure deficit control in relation to water transport and water productivity in greenhouse tomato production during summer. Scientific Reports, 7(1).